Superheterodyne receiver



Dec. 10, 1-946. H. J. BENNER 3 6 4 SUPERHE'IERODYNE RECEIVER Filed Nov. 10, 1938 AVG 7b Osc/LM 70B Tums/a INVENTOR wmqead fiav/vse Patented Dec. 10, 1946 UNITED STATES PATENT OFFICE SUPERHETERODYNE RECEIVER Howard J. Banner, Springfield, Mass, assignor of one-half to Colonial Radio Corporation, Buffalo, N. Y., and one-half to The F. W. Sickles .Company, Springfield, Mass.

Application November 10, 1938, Serial No. 239,788

, 16 Claims. 1

This invention relates to radio receiving apparatus, and more particularly to signal selecting circuits for superheterodyne receivers,

In such receivers, as is well known, the intermediate frequency amplifying stage or stages are tuned to a frequency above audibility and usually below the lowest frequency of the broadcast band. Incoming radio frequency signals are heterodyned or beat with an oscillation produced by the local oscillator, and this gives rise to oscillations of a frequency which may be either the sum of or the difference between the incoming frequency and that of the local oscillator. Only those oscillations thereby produced which have a frequency enabling them to pass through the intermediate frequency amplifier are then amplified.

For example, if the intermediate frequency amplifier is tuned to 400 kilocycles and a 550 kilocycle signal is being received, and the local oscillator has, when this particular signal is being received, a frequency of 950 kilocycles, then there will be produced a fbeat frequency of 400 kilocycles. However, if there is also an incoming signal of 1350 kilocycles at the same time, a new oscillation will be produced by the heterodyne action between the local oscillation and the 1350 kilocycle signal, and this new oscillation will likewise have a frequency of 400 kilocycles and will accordingly pass through the intermediate frequency amplifier, resulting in interference and cross-talk with the desired signal. The frequency of the undesired signal is termed the image frequency, and it is equal to the frequency of the desired signal plus twice the intermediate frequency. 2

It is usual to provide a signal selecting circuit in advance of the first detector, and this circuit is ordinarily arranged to be tuned to the frequency of the desired signal. However, even when such a circuit is used, image interference may still be encountered and it has been proposed in the past to eliminate or reduce this image interference by providing preselecting circuits and the like, and also by providing one or more wave traps associated with the antenna and variably tuned.

The employment of wave traps for image suppression increases the cost of the receiver, since it is necessary to provide an additional coil and an additional condenser, one or the other of which is usually variable. The provision of such parts increases the cost of the receiver, from the standpoint of material and parts employed and also the labor of assembly,

I have discovered that it is possible to provide a simple tuning circuit with only one variable tuning reactance which acts simultaneously as a selector of desired frequencies and as an image suppressor; that is to say, the variation'of the variable reactance causes the circuit to select the! desired signal, at the same time to present an extremely high impedance to the image signal, and I have found that circuits according to my-'inven tion produce signal to image ratios as high 'as, and in many cases considerably higher than, the best signal to image ratios produced by conven-- tional wave trap circuits. As an example, employing the single tuned circuit of my invention, I have been able to obtain signal to image ratios as high as 70,000. 7

Among the objects of my invention are: H

To provide an image suppression circuit which does not require separate wave trap or preselector circuits;

To provide a circuit in which the image suppression is automatically performed by the main tuning circuit,

Still another object of my invention is to provide a simple signal selecting circuit which will' deliver signal to image ratios as high or'higher than those obtained from conventional wave trap or preselector image suppressors.

Still other objects of my inventionwill be apparent from the specification,

In this application I have particularly pointed out and distinctly claimed the part, improvement or combination which I claim as my invention of discovery and I have explained the principles thereof and the best mode in which I have'con templated applying those principles so as to distinguish my invention from other inventions.

In the drawing- The figure is a circuit diagram of one form of circuit in accordance with my invention.

In the practice of my invention, for some pur-. poses I prefer to employ what is known as permeability tuning, although this invention is not limited thereto. If such permeability tuning is employed, the variation in resonant frequency is obtained, not by rotation of a variable condenser, but by movement of a core of magneticmaterial in the field of an inductance. Such cores are satisfactorily formed of iron dust mixed with a suitable insulating and binding material and pressed or otherwise processed to form what appears as a solid bar.

Since they are easily obtainable on the marke invention, they are not described in detail.

If such a core is moved axially into and outof a solenoid in a resonant circuit, there is obthis change of resonant frequency being due, to a very considerable extent, to the variation in the value of the inductanceof' the" solenoid with the positionof the: core, and probably also, in some. arrangements, to some extent at least, to the variation of the capacity of the circuit resulting; from change of position of the core,..

Referring more particularly to the figure; I represents an antenna of any suitable typew,

which may be connected to groundz'througni coupling condenser 2. C'urrnntsprodirceidihtheantenna I by incoming signals will flow to some extent through inductance 3, shunted by condenser 5, and the voltage thereby produced may be supplied to the grid 9 of the first. tube 'l,..

construction of the superheterodyne beyond this point, and the same is well known in the'art, only the first tube 1 is indicated.

The desiredsignals may be selected by movement of the core 4 into and out of the field of coil 3, and at the same time the frequency of the local oscillator is: varied in such" 'a manner that the signalselected in the antenna. circuit by movement of the coil 4 is heterodyned to-pro d-ucev oscillations of the desired intermediate fr quency.

In adjusting the circuit for operation, the condenser. 5: may be omitted and the desired number of turns in. inductance 3 provided so that movement of? the core 4 causes the circuit to i tune over the range desired, in this instance from 500 to 1500 kilocycles, and the tuning. circuit formed by-inductance- 3-.and its associated elements, herein called the signal tuning circuit, is-preferably ftracked withtheoscillator tuner bytrinnner condenser-6. so that thereceiver may be operated: bya single control as indicated by theconnected arrowsin the drawing. The circuit.

will. now operate as aconventional superheter ody'neand imageinterferencemay be experienced.

If now condenser 5 be connected across coil 3 and chosen of the proper value, a very great immovement in signal: to image ratio is noticed. In choosing the value of condenser 5, it is picked of such magnitude as to cause the inductance 3 to resonate with condenser 5 at the image frequency (which, as stated, is equal to the desired signal frequency, plus twice the intermediate frequency) whengthetuning circuit is tuned to a desired signal frequency;

The connection of thiscondenser, which will order of. two to twelve mmf., may slightly disturb:

1 the tracking of the main tuning circuit with the i oscillator tuner, in which case: it may be neces-v sary to retrim the main tuning circuit by variation' of. the trimmer condenser 6, but it will be found that when the operation is completed the entire input. circuit; as a whole, is resonant fethedesired'signal frequency; whereas, inductance ordinarily be quite small, for instance of the 1 of my invention, as will beelear to those 3 and condenser. 5, forming a closed loop circuit,

are resonant to the image-producing signal and cause extreme attenuation thereof while not interfering with the selection by resonance of the desired signal. I have found that movement of thecore 4 into and out of the coil 3 varies the tuning of the entire circuit and: also, the; tuning of the'imag suppressor 3+5. in a satisfactory approximation to the desired manner.

"In order for the signal or main tuning circuit to vary from 500 to 1500 kilocycles, i. e., 3 to 1 ratio; the inductance of that circuit must vary in a. 9. to 1 ratio. It will be noted, however, that. while the resonant circuit varies from 5037 to. 150.0 kilocycles, v the image frequency varies from. 1460. tn 2510} kilocycles when the intermediate frequency is455 kilocycles. In this case the ratio of frequencychange is 1.72 to 1 and the changeiwofiinductance required to make the image circuit track exactly should be in th square of this ratio, or. 2.96-1.

From. this it willbeseen that. if the main tuning circuit is tuned to the lowest broadcast frequency, i. e., 550. kilocycles, andthe. image circuit 3-.51 then tuned, by varying.v condenser 5,. to. the. image. frequency,.then as; the. main tuning circuit. frequency is increased by movement. ofthe core 4. out of the inductance,.the. reduction of; inductance in the image trap proceeds too fast, and. the. resonant? frequency of the. image circuit. rises above the image'frequenc'y.

This. departure from. the theoretical image. value is, in most cases, not-a serious. matter, .be-

- cause, ordinarily, themost harmful-image. inter f'erence is. obtained when. the. main-tuning circuit istuned to thelow frequency end of. thebroadcast band.

'It may be noted that under certain. conditions the value of condenser "2. may be. zero; that. is, that physical condenser may be omitted; One. instance of this is in automobile radio installations where the. running-board's are used as an. aerial. whichis in parallel with condenser 2 is so large, comparatively, that condenser 2, may beomitted.

Condenser 2. may be'omitted with an aerialof small capacity also; such as av fishpole. In this-instance condenser 6- may be adjusted. to compensate: for'the variationin antenna capacity; I r V While I have shownanddescribed certain pre-. ferred embodiments of my invention, it? will. be apparent that modifications. and changes may be. made. without departing from the spirit and scope. skilled.

in the art. I

I claim: l g,

1; In-a superheterodyne receiver, incombina-r tion, inductance and capacity elements: associ: ated. together to form a resonant circuit, tunments to select desired. signal. frequencies, said variable element forming, with another of said.

elements, a parallel tuned. resonant circuit which is maintained resonant to the image ofthe SE:

lected signal frequency I v 2. In a superheterodyne receiver, in. combination, inductance and capacity elements associ ated togetherv to form a resonant circuit, tunable as. a whole by variation of said inductance to select; desired signal frequencies/said inductanceforming with a capacity element a secondresonant circuit parallel tuned. to the image ofthe selected signal frequency; 7 r

3. In a superheterodyne. receiver, inzcombina In this. instance, the antenna capacity;

tion, inductance and capacity elements associated together to form a resonant circuit tunable as a whole to select desired signal frequencies, a core of magnetic material movable into the, field of said inductance to tune said circuit, said inductance having a capacity connected thereto and forming therewith 'a second resonant cirf cuit maintained parallel resonant to the image of the selected signal frequency.

4. In a superheterodyne receiver, in combination, inductance and capacity elements associated togetherto form a resonant circuit, tun-' able as a whole by variationof said inductance to select desired signal frequencies, and a cir cuit comprising a condenser in shunt with said inductance, the circuit formed byv said inductance and said condenser in shunt therewith being parallel tuned to the image of the selected signal frequency.

5. In a superheterodyne receiver, a combined signal selecting and image rejecting circuit comprising a plurality of reactance elements forming a resonant circuit, at least one of said reactances being variable to select signals of a desired frequency, said variable element being shunted by an additional reactance of opposite sign of such value as to constitute a second resonant circuit parallel resonant to the image of the resonant frequency of the first resonant circuit, and means for varying the value of said variable reactance to simultaneously vary both resonant frequencies while maintaining the second at the image value of the first.

6. In a superheterodyne receiver, an input tuning circuit comprising a pair of capacity elements, a connection between said capacity elements, said connection including, in series, a closed tuned circuit, the inductance and capacity of said closed tuned circuit being so chosen that said closed tuned circuit is resonant to the image frequency, and one of the reactance elements of said closed tuned circuit being variable over a range such as to tune the input tuning circuit as a whole to desired signals while maintaining said closed circuit resonant to said image frequency.

7. In a superheterodyne receiver, an input tuning circuit comprising a pair of capacity elements, 9, connection between said capacity elements, said connection including, in series, a closed tuned circuit, the inductance and capacity of said closed tuned circuit being so chosen that said closed tunedcircuit is resonant to the image frequency, and one of the reactance elements of said input tuning circuit being variable over a range such as to tune the input tuning circuit as a whole to desired signals while maintaining said closed circuit resonant to said image frequency.

8. In a superheterodyne radio receiving circuit, an audion tube having a control grid, a selector circuit comprising an inductance coil, a first capacitor across the ends of said inductance coil, a second capacitor in series with the inductance coil at one end, an impedance in series with the inductance coil at the other end, the opposite ends of the impedance and capacitor being grounded, means for impressing a received signal across the terminals of the im pedance, the second capacitor and inductance coil being connected to the grid, and means for varying the inductance of the coil to tune the circuit as a whole to a desired frequency and to tune the image trap composed of the inductance coil and the first capacitor to a frequency subtwice the intermediate frequency, I v

9. In ajsuperheterodyne radio' re ceivingi circuit, an audion tube having a control grid,'-a selector circuit comprising an inductance coil, a first capacitor across theends of said inductance coil, a second capacitor in series with the inductance coil atone end, an impedance in series with the inductance coil at the other end, an aerial operatively connected to the inductance coil and to the impedance, the opposite ends of the impedance and capacitor being grounded, means for impressing a received signal across the terminals of the impedance, the second capacitor and inductance coil being connectedto the grid, and-means for varying theinductance of the coil to tune the circuit as a whole to a desired frequency and to tune the image trap composed of the inductance coil and the first capacitor to a frequency substantially equal to the desired frequency plus twice the.interrne'- diate frequency. j

10. An image suppression system'for a super heterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a second resonant circuit in series between said antenna and said control electrode, said resonant circuits being tuned by inductance variation respectively to a signal frequency and to the corresponding image frequency.

11. An image suppression system for a superheterodyne radio receiver having an antenna stantially equal to desired 5 frequency plus and a vacuum tube with a control electrode, in-

eluding a first resonant circuit a portion of which is connected between said control electrode and ground, and a second resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common a variable inductance device comprising an inductance coil and a ferromagnetic core movable relatively thereto and being tunable simultaneously over substantially different frequency ranges by motion of said core.

12. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a second resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common a variable inductance device comprising an inductance coil and a ferromagnetic core movable relatively thereto, said resonant circuits being simultaneously tuned by motion of said core respectively to a signal frequency and to the corresponding image frequency.

13. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising an inductance coil and a movable ferromagnetic core, first and second capacitors connected with said inductance device to secure resonance within a first range of frequencies by movement of said core, and a third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies by movement of said core, said capacitors being so chosen that any frequency of said second range is higher than the corresponding frequency of said first range by a practically constant amount throughout said core movement.

itors in series shunted across said inductance device to secure resonance within a first range of frequencies by movement of said core, and a third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies by movement of said core, said capacitors being so chosen that any frequency of said second range is higher than the corresponding frequency of said first range by a prac-' tically constant amount throughout said core movement. I t s n 15, In a superheterodyne radiovreceiv'ing syse tem, an antenna, a vacuum tube having an inputelectrode, and, elements including a variable inductancedevice comprising an inductance coil and a movable ferromagnetic core, a first resonant circuit including said inductance device and tunable thereby over a first ran e of. frequencies connected between said antenna and V8 ground, anda secondresonant circuit including said. inductance device and tunable, thereby over a second frequency. range in series between "said antenna and said input electrode, said elements being so proportioned that said resonant circuits are maintainedat a practically constant frequency difference during movement of said core. a V v '1 16. In a superheterodyne receiver, in combi nation, an inductance element anda plurality of capacity elements connected to et er to form a single resonant signal selector circuit for connection at one pointto an antenna and at another point to the control electrode'of a thermi ionic tube, said inductance having one of said capacity elements connected in shunt therewith and forming with said inductance a resonant circuit parallel tuned to the image of a, selec-V table signal frequency, said parallel tuned cit cuit being interposed in series between the points of connection to said antenna and to said control electrode. I v

HOWARD J. BENNER, 

